In the world of radio amateurs, especially during repairs and building a new devices, you should have a device for measuring of capacitance and inductance, this device is called LC meter. Today, on the Internet, you can find many schemes of such devices, complex and very expansive. But you can create your own LC meter. Almost all LC meter circuits using microcontrollers presented on the Internet look the same. The idea is to calculate the nominal value of unknown components using the formula that depending on frequency , capacitance and inductance. For simplicity of its design, we will use the internal comparator of the microcontroller as a generator. To display the information a LCD from the Nokia 3310 has been used with a resolution of 84×48.
The schematic of the LC meter based microcontroller
The Printed circuit board PCB
A- SMD version 1
B- SMD version 2
C- DIP version 1
The heart of the device is the PIC18F2520 microcontroller . For stable operation of the generator, it is better to use non-polar capacitors or tantalum capacitors as C3 and C4. You can use any relay that is suitable for voltage (3-5 volts), but preferably with the lowest possible contact resistance for the closed position. For sound, a buzzer is used without a built-in generator, or a regular piezoelectric element.
When you start the assembled device for the first time, the program automatically starts the display contrast adjustment mode. Use the 2/4 buttons to set the acceptable contrast and press the OK button (3). After performing these steps, the device should be turned off and on again.
For some tuning of the meter operation, the menu has a “ Setup ” section . In the “ Capacitor ” submenu , you must specify the exact value of the used calibration capacitor (C_cal) in pF. The accuracy of the indicated rating directly affects the measurement accuracy. It is possible to control the operation of the generator itself with the help of a frequency meter at the control point “B”, however it is better to use the already built-in frequency control system in the “ Oscillator ” submenu .
With the selection of L1 and C1, it is necessary to achieve stable frequency readings in the region of 500-800 kHz. A high frequency has a positive effect on the accuracy of the measurement, while the frequency of the generator may deteriorate with increasing frequency. The frequency and stability of the generator, as I said above, is conveniently monitored in the ” Oscillator ” menu section . If you have an external calibrated frequency meter, you can calibrate the frequency meter of the LC meter. To do this, you need to connect an external frequency meter to the control point “B” and use the +/- buttons in the “ Oscillator ” menu to select the constant “K” so that the readings of both frequency meters coincide. For the correct operation of the power battery status display system, it is necessary to configure a resistive divider built on resistors R9, R10, then set the jumper S1 and write the values in the fields of the Battery section.
- Measure the supply voltage of the microcontroller (pins 19 – 20). This is the “V.ref” voltage reference
- Measure voltage to resistive divider = U1
- Measure the supply voltage after the divider = U2
- Calculate the divisions “C.div” = U1 / U2
- Enter the received numbers in the appropriate sections of the menu, saving them by pressing the “OK” button.
Also add the voltage “V.max” – the maximum voltage of the power battery (all segments of the displayed battery are filled) and, accordingly, “V.min” – the minimum voltage of the power battery (all segments of the battery are extinguished, the device signals the necessary change or charge of the power battery). The values of the supply voltage for displaying intermediate segments on the battery icon will be calculated automatically after entering information about “V.max” and “V.min”.
Work with the device
This LC-meter menu contains sections Light , Sound , Memory . In the Light section , you can enable or disable the LCD backlight. Sound section , for on / off sound. In the Memory section, you can see the results of the last 10 measurements, and also (for beginners) see the result in different units of measurement. The buttons are described by the icons located at the bottom of the screen.
- ( F ) – “Function” switch to the Setup menu
- ( M ) – “Memory” storing the measurement results in memory
- ( ☼ ) – “Light” on / off the backlight
- ( C ) – “Calibration” calibration
The main screen contains a conditional scale of measurement error, which must be controlled and, if necessary, calibrated in a timely manner.
1. Switch the device to capacitance measurement mode. Perform calibration. Make sure that the measurement error is within acceptable limits. In case of large deviations, repeat the calibration.
2. Connect the unknown capacitor to the terminals. The measurement result appears on the screen. To save the result in memory, press (M).
1. Switch the device in the inductance measurement mode. Close the terminals. Perform calibration. Make sure that the measurement error is within the acceptable limits. In case of large deviations, repeat the calibration.
2. Connect the unknown inductance to the terminals. The measurement result appears on the screen. To save the result in memory, press (M).
LC Meter operation video